1. Field of the Invention
The present invention is related to balancing/unbalancing structures, or “baluns,” for use in gigahertz wireless applications with multiple frequencies of operation.
2. Related Art
There is an increasing demand for wireless devices that are capable of communicating in multiple frequency bands. For example, a wireless device configured for the United States and European markets may require the ability to operate in four bands: the European cellular telephone band (880-960 MHz), the United States PCS band (1850-1990 MHz), the Bluetooth band (2.4-2.5 GHz) and the 802.11a unlicensed band (5.15-5.25 GHz).
A balun (short for BALanced to Unbalanced) is a transformer connected between a balanced source or load (signal line) and an unbalanced source or load (signal line). A balanced line has two signal line conductors, with equal currents in opposite directions. The unbalanced signal line has just one conductor, where the current in it returns via a common ground or earth path. Typically, an RF balun function is implemented as an off-chip transformer or as a quarter wave hybrid (lumped or microstrip) integrated into an RF circuit board.
RF wireless circuits utilize balanced outputs of signals to minimize the effect of ground inductance and to improve common mode rejection. Circuits that benefit from balanced operation include mixers, modulators, IF strips and voltage controlled oscillators. These balanced outputs, moreover, consist of differential signals which must be combined to provide a single ended output signal. Thus, a balun is a RF balancing network or electric circuit for coupling an unbalanced line or device and a balanced line or device for the purpose of transforming from balanced to unbalanced or from unbalanced to balanced operation, with minimum transmission losses. A balun can be used with an unbalanced input and a pair of balanced outputs or, in the reverse situation, a pair of balanced sources and an unbalanced load. Baluns can be used to interface an unbalanced input with a balanced circuit by dividing the signal received at its unbalanced terminal equally to two balanced terminals, and by providing the signal at one balanced terminal with a reference phase and the signal at the other balanced terminal with a phase that is 180° out-of-phase relative to the reference phase. Plus or minus 180° baluns can be used to interface a balanced or differential input from a balanced port of a balanced circuit providing output signals which are equal in magnitude but 180° out-of-phase and an unbalanced load driven by a single-ended input signal. The balun combines the signals of the balanced input and provides the combined signal at an another port.
A 180° hybrid device is constructed from several sections of quarter-wavelength transmission lines and a section of half-wavelength transmission line. The drawbacks of the 180° hybrid device are larger size, difficulty in achieving a high impedance transformation ratio, and limitation to a balanced pair of unbalanced outputs.
A particular problem that exist in the context of multi-frequency operation is the interference between the various bands. For example, a device that needs to operate in both the unlicensed band (5.3 GHz) and the Bluetooth band (2.4-2.5 GHz) will experience interference from the other band. This is illustrated in FIG. 1.
As shown in FIG. 1, the RF device includes power amplifiers 101 with differential outputs for 2.4 GHz operation, a 180° hybrid balun 102 for converting to single ended signal, and an antenna 103 that transmits at 2.4 GHz. The RF device also includes a power amplifier 104 with differential output for 5.3 GHz operation, a 180° hybrid balun 105, and an antenna 106. The 5.3 GHz antenna 106 will receive signals from the 2.4 GHz antenna 103, causing interference and cross-talk in the 5.3 GHz circuitry from 2.4 GHz signal.
Accordingly, a need exists for a balun circuit that occupies a minimal amount of space that would filter out undesirable interference and cross-talk from other bands of operation.